Various sensors are known in the pressure sensing arts. Pressure transducers are well known in the art. One example of a pressure transducer is a device formed with a silicon substrate and an epitaxial layer, which is grown on the substrate. A portion of the substrate can then be removed, leaving a thin, flexible diaphragm portion. Sensing components can be located in the diaphragm portion to form a pressure transducer. In operation, at least one surface of the diaphragm can be exposed to a process pressure. The diaphragm deflects according to the magnitude of the pressure, and this deflection bends the attached sensing components. Bending of the diaphragm creates a change in the resistance value of the sensing components, which can be reflected as a change in the output voltage signal of a resistive bridge formed at least partially by the sensing components.
Some techniques for forming a composite diaphragm for a pressure transducer or similar device involve configuring a substrate layer having a first conductivity type, wherein the substrate layer includes a first surface. Positive implants can then be deposited in the first surface of the substrate layer, and an epitaxial layer grown on the first surface of the substrate layer so that the positive implants form positive diffusions in the epitaxial layer. An oxide pattern can be then formed on the epitaxial layer, and a top layer deposited over the epitaxial layer and oxide pattern. The substrate layer and positive diffusions of the epitaxial layer can then be etched to form the composite diaphragm. Such a composite diaphragm can therefore be provided for use in a pressure sensor or like device. The diaphragm comprises a first layer of silicon nitride and a second layer attached to the silicon nitride layer and comprising a pressure sensor pattern of silicon material.
Pressure transducers of the type which comprise a thin, relatively flexible diaphragm portion of suitable material, such as silicon or ceramic, on which either a selected resistive element or a capacitive plate is printed whereby exposure to a pressure source causes deflection of the diaphragm will cause a change in the resistive value of the resistive element or a change in the spacing of the capacitive plate with a mating capacitive plate and concomitantly a change in capacitance are therefore well known in the art.
An example of a transducer configuration is disclosed in U.S. Pat. No. 6,945,118, entitled “Ceramic on Metal Pressure Transducer,” which issued to William D. Maitland, Jr. on Sep. 20, 2005 and is incorporated herein by reference. U.S. Pat. No. 6,945,118 generally discloses a transducer apparatus and a method of forming the transducer apparatus. A metal diaphragm is molecularly bonded to a ceramic material to form a ceramic surface thereof. A bridge circuit is then connected to the ceramic surface of the metal diaphragm. An input pressure port for pressure sensing is also provided in the configuration of U.S. Pat. No. 6,945,118, wherein the input pressure port is connected to the metal diaphragm to thereby form a transducer apparatus comprising the metal diaphragm, the bridge circuit and the input pressure port.
In the example of U.S. Pat. No. 6,945,118, the metal diaphragm is welded to the input pressure port. The metal diaphragm and the ceramic surface thereof preferably operate over a temperature of range of at least approximately −40° C. to 150° C., as does the transducer apparatus. The ceramic material is molecularly bonded to the metal diaphragm to form the ceramic surface thereof. The ceramic surface bonded to the metal diaphragm can also be configured as a ceramic substrate. The ceramic surface provides corrosion protection to the metal diaphragm. The bridge circuit generally comprises a resistor network and provides an output proportional to the applied force. A flex circuit comprising an ASIC (Application Specific Integrated Circuit), associated circuitry and EMI protection provides signal conditioning, calibration and compensation. A snap on connector system comprising a plastic snap on lead frame and Z axis conductor material can be utilized for connecting the flex circuit to the bridge network which is located on the diaphragm.
When used as a low pressure sensor, the economical packaging of the transducer in a housing can provide an effective seal. Preventing related to the mounting and sealing of the transducer from influencing the output, however, can become problematic. This is caused, at least in part, by the significant difference in thermal expansion between the material used to form the transducer, e.g., silicon, ceramic or the like, and the housing of plastic or the like. A conventional sealing arrangement involves placement of a ring of sealing material around an inlet pressure port in a housing and mounting the transducer so that the pressure sensitive diaphragm is precisely aligned with the pressure port. This conventional arrangement not only involves stress isolation issues, it also limits flexibility in design choices in defining the location of the transducer within the package.
FIG. 1 illustrates a pictorial diagram of the prior art welding of a diaphragm 102 to a port 104. FIG. 2 illustrates an isometric view of the prior art connection of the diaphragm 102 to port 104. FIG. 3 illustrates a cross sectional side view of the diaphragm 102 and port 104 depicted in FIGS. 1-2. Note that in FIGS. 1-3 identical or similar parts are generally indicated by identical reference numerals. The diaphragm 102 welded to port 104 is not sufficient for extreme pressure conditions, such as, for example, environments operating under pressure of up to 3000 Bar (i.e., 43,500 psi). The integrity of a welded construction such as that depicted in FIGS. 1-3 is a source of reliability concern during validation life testing. The style of sensor design depicted in FIGS. 1-3 is thus not adequate for ultra high pressure applications.
One of the major problems with pressure transducer devices, including those that utilize diaphragm or diaphragm portion configurations, is that such devices are not reliable in corrosive and high-temperature applications. A need therefore exists for a low-cost high accuracy pressure transducer that can be used in corrosive media and high-temperature applications.